1. Field of the Invention
The present invention relates to a raising and lowering mechanism for blinds, such as venetian blinds, roman shades, pleated blinds or the like. It includes a drive shaft and a driven cord spool mounted in keyed connection with the drive shaft, about which a lift cord can be wound in order to raise or lower the blind.
2. Description of the Relevant Art
Such devices are known and described in e.g. GB 986,529, U.S. Pat. No. 3,181,595 EP 0,554,212, CH 581,257, DE 16269 and GB 2,333,314.
A rotatably driven spool is used to wind or unwind the lift cords of a blind. The spool is usually mounted in a keyed connection to a driven shaft. The shaft can be driven by a pulley and chain or by a motor (not shown). In order to ensure even, regular windings of the lift cord, the spool can be provided with means to ensure that the spool is displaced longitudinally during rotation. Such a solution is described in GB 986,529, where the spool is provided with a screw thread, which is in driven connection to a screw thread of one of the journals in which the spool rests.
Other solutions are the use of a circumferentially threaded spool as described in U.S. Pat. No. 3,181,595. The cord winds in circumferential threads and the spool is transported by the threads in its longitudinal direction. The thread at the same time prevents overlapping windings. A drawback of such a spool was that different sizes of cords needed a differently configured spool thread. Also both these prior art solutions require relatively expensive machining of parts and are complicated in design and also require a lot of longitudinal space.
In order to solve the problem of transporting the windings in longitudinal direction without overlapping and without threading the spool, conical cord spools were proposed in a number of variations. These conical spools usually have a first end having a first diameter and a second end having a second diameter, the second diameter being smaller than the first. The spool includes a sloping portion where, the diameter of the spool reduces from the first diameter over a predetermined longitudinal length to the second smaller diameter. The cord is affixed to the small diameter end and is guided onto the spool at the large diameter end. Such spools are described in DE 16,269, CH 581, 257 and EP 0,554,212. The cord is wound around the spool starting at the first end of the spool with the larger diameter and is transported along the sloping part to the second end of the spool with the smaller diameter by each next winding. The decrease in diameter of the spool in the sloping portion ensures the transport of the windings and the even, regular winding of the cord. EP 0,554,212 additionally includes a circumferential flange or shoulder means at the larger diameter end of the spool to ensure transport of the windings in longitudinal direction towards the small diameter end of the spool. Also all the conical spools have a possibly smooth surface on the spool, to reduce friction and to facilitate the sliding of the windings in longitudinal direction towards the smaller end.
It has proven difficult to design a conical cord-winding spool, which ensures good winding under all circumstances. The problem of the longitudinal transport of the windings was largely solved by the conical shape, the shoulder means and by providing a smooth surface. Nevertheless an additional problem has sometimes manifested itself. By using a conical shape and a smooth surface to ensure the longitudinal transport of the windings, the friction of the cord in the circumferential direction also became very low and was no longer sufficient to reduce the tension of the cord over the windings. So although a low friction in longitudinal or axial direction is desired, a too high tension of the cord in the circumferential direction of the cord spool adversely affects the transport of the lift cord in axial direction of the spool.
It has now been found that the use of longitudinally extending ribs on the spool solves this problem.